Low noise air distributor



Oct. 23, 1962 B. E. CURRAN ETAL 3,059,564

LOW NOISE AIR DISTRIBUTOR Filed Oct. 30, 1959 2 Sheets-Sheet 1 IN VEN TORS BERNARD E. CURRAN GLENN E. KAUTZ BY THEODORE w. MARSHALL up? A oRNEY Oct. 23,. 1962' B. E. CURRAN ETAL 3,059,564

LOW NOISE AIR DISTRIBUTOR 2 Sheets-Sheet 2 Filed Oct. 50, 1959 o o O o o 8 7 6 5 4 600 I200 SOUND FREQUENCY, CYCLES PER SECOND SOUND FREQUENCY, CYCLES PER SECOND O O O O O 8 7 6 5 4 5980 cEmzEE ozzow L L m M NN 8 EM R ZA Y WW E U N A R K 0 O .E "H w DER A 4 R 0 ND NNO fi w BGT 4 2 I50 SOUND FREQUENCY, CYCLES PER SECOND 0 0 0 0 0 0 8 3 JUQQMQ mzm. .z ozzow United States Patent The present invention relates to air distributing means having a low noise level.

In the ventilation art, air is supplied through duct work into rooms of buildings for distribution. Noises accompany the release of air in the individual rooms. These noises include sounds of a wide frequency spectrum including low frequency sounds and high frequency sounds.

Noises in the audible sound range from about 15 cycles per second to about 20,000 cycles per second become objectionable to the occupants of the rooms. Noises arise in ventilation systems from a variety of sources including fan noises, mechanical vibrations, duct resonance, air turbulence, direction changes in the air flow, duct oscillations, air velocity effects, and the like. All of these noises blend together into a spectrum of sounds which are presented at the air distribution point. It is a commonplace practice to cover the surfaces of air discharge apparatus with fibrous materials capablev of absorbing a portion of the energ of relatively high frequency sound waves.

While sound absorbing coatings are effective in lowering the intensity of the relatively high frequency sounds, the coatings are ineffective in reducing the intensity of the relatively low frequency sounds. According to the present invention ventilating gases are passed initially through a permeable obstruction havin a multiplicity of gas passageways therethrough. Each gas passageway has a cross-section which is negligible in contrast to that of the duct in which the ventilating gases are flowing. The cumulative cross-sectional area of said gas passageways is between about 50 and about 75 percent of the cross-section of the duct in which the gases are flowing. The function of the permeable obstruction is to convert the energy of the relatively low frequency sounds into energy having a relatively higher frequency. In the higher frequency form, the sound energy can be absorbed by the conventional sound absorbing linings.

When the ventilating gases, having passed through the permeable obstruction, subsequently impinge against a conventional sound absorbing lining, sound energy is absorbed from the relatively high frequency sounds which were originally present as well as from the relatively high frequency sounds resulting from conversion of the relatively low frequency sounds originally present. As a result, the over-all noise intensity presented in the region of the gas distributor can be maintained at an acceptable level in the audible sound range to minimize the discomfort of the room occupants.

The principal object of this invention is to'provide a method and apparatus for minimizing the level of audible noise accompanying discharge of ventilating gases into a room.

A further'object of this invention is to provide a method and apparatus for converting a portion of the energy of the relatively low frequency sounds associated with a flowing stream of ventilating gas into relativelyhigh frequency forms of sound energy and thereafter absorbing a portion of the relatively high frequency sound energy associated with the gas stream in its altered condition.

These and other objects and advantages of the present invention will become apparent from the following detailed description by reference to the accompanying drawings in which: 1 I 7 FIGURE 1 is a schematic illustration in cross-section ice showing the present invention in its simplest embodiment;

FIGURE 2 is a cross-section illustration of a ventilation distributor adapted to practice the present invention;

FIGURE 3 is a cross-section illustration of a preferred embodiment of a ventiiation mixing box for blending and discharging separate streams of ventilation air into a room;

FIGURE 4 is a cross-section illustration of the ventilation mixing box taken along the line 44 of FIGURE 3;

FIGURES 5, 6 and 7 are graphical representations of various relationships between sound intensity and sound frequency which illustrates certain properties of the present invention;

FIGURE 8 is a view of an enlarged scale illustrating the structure of a preferred permeable obstruction; and

FIGURE 9 is a cross-sectional view, on an enlarged scale, illustrating the connection between a pressure sensing conduit and a filter shown in FIGURE 4.

Referring to FIGURE 1 there is illustrated a duct 10 through which gases, indicated by the arrow A, are flowing for discharge into an enlarged chamber such as room as indicated by the arrow B. Associated with the flow of gases at A is a noise phenomenon including sounds ranging through the entire spectrum of frequencies included in the audible sound range.

A permeable obstruction II extends across the duct It? in the path of the flowing gases A. The permeable obstruction 11 has a multiplicity of gas passageways extending therethrough, each gas passageway having a crosssection which is negligible in contrast to that of the duct 1t). Suitable materials for the permeable obstruction 11 include a perforated plate of solid material such as metal or plastic, a filamentary screen such as wire mesh or plastic filament mesh, a batt of coarse fibrous materials such as straw, coconut fiber, steel wool, or similar filamentary material. The cumulative crosssectional area of all of the said gas passageways should be from about 50 to about percent of the cross-section of the duct It.

In traversing the permeable obstruction II, the gases A experience a measurable pressure drop, preferably slight. The spectrum of sounds which are airborne by the flowing gas stream experiences a pecular frequency shift. The relatively low frequency sounds are in part converted to relatively high frequency sounds. The relatively high frequency sound waves are virtually unaffected by the passage of the gases through the permeable obstruction 1i. Downstream from the permeable obstruction 11 the gases impinge against a duct lining 12 having sound absorbing properties. The airborne sounds of relatively high frequency impinge against the sound absorbing lining 12. The residual relatively low frequency sounds are virtually unaffected by the sound absorbing lining 12. Thus the noise spectrum associated with the discharging gases B has a lower intensity of relatively low frequency sounds and of relatively high frequency sounds than the noise spectrum associated with the flowing gases at A.

In the absence of the permeable obstruction 1-1, the sound absorbing lining 12 would serve to absorb sound energy originally present as relatively high frequency sounds which are airborne by the flowing gases at A. In the absence of the sound absorbing lining 12, the permeable obstruction 11 would serve merely to convert some of the sound energy of the relatively low frequency sounds to sound energy in the form of relatively high frequency sounds which would pass unabsorbed into the enlarged chamber at B. Thus it is the sequential combination of the permeable obstruction 11 and the sound absorbing lining 12 which forms the improvement of the present invention.

Where the cumulative cross-sectional area of the gas passageways in the permeable obstructions is less than about 50 percent of the cross-section of the duct in which the obstruction is placed, there is excessive pressure drop in the gases flowing through the obstruction. For example, a sponge which possesses a highly developed independent pore structure will present few gas passageways extending through a section of the material.

Where the cumulative cross-sectional area of the gas passageways exceeds about 75 percent of the cross-section of the duct, the desired optimum reduction in noise level is not attained.

Referring to FIGURE 2, there is illustrated a preferred mixing and discharge outlet unit in which the discharging gases pass through a tortuous path prior to discharge into a room whereby greater absorption of the sound energy of the relatively high frequency sounds is assured. A duct conveys ventilating gases C for discharge into a room as indicated by the arrows D. A permeable obstruction 16 extends across the duct 15 as previously described. The duct 15 communicates with a mixing chamber 17 having a grid outlet 18. Mounted within the chamber 17 is a baflle 19 positioned in direct line between the grid opening 18 and the duct 15. The walls of the chamber 17 as well as the baffle 19 are covered with a lining 20 of sound absorbing material such as bat-ts of glass Wool or mineral wool. By providing the bafiie 19 in the direct line of flow between the duct 15 and the grid opening 18, the discharging gases after passage through the permeable obstruction 16 are required to follow a tortuous path prior to discharge. The probability of impingement against the sound absorbing lining 20 thereby is enhanced.

As before, the permeable obstruction 16 is positioned in the line of gas flow upstream from the sound absorbing linings. The intensity level of airborne noises associted with the gases discharged at D is lower than the intensity level of the airborne noises in the flowing gases C both in the relatively high frequency ranges and in the relatively low frequency ranges.

The present invention finds particular utility in mixing and distribution boxes for dual duct air conditioning systems of the type disclosed in co-pending application S.N. 755,048 (now abandoned) by Bernard B. Curran, filed August 14, 1958. A typical dual duct air conditioning mixer box and discharge unit is illustrated in cross-section in FIGURE 3. A conduit is provided to deliver warm air from a central source to a mixing box 32. A conduit 31 is provided to deliver cool air from a central source to the mixing box 32. The mixing box 32 is a generally rectangular structure having vertical sidewalls 33 and a horizontal top wall 34 including a grid opening 35. The air supply ducts 30 and 31 preferably are mounted below the level flooring 36. Within the mixing box 32 is a horizontal baffle plate 37 serving as a deflector to direct the passage of air around its edges as indicated by the arrows E. Within the mixing box 32, control chimneys 38 are connected to each of the air ducts 30 and 31. Each control chimney 38 has vertical sidewalls 39 and a rectangular cross-section. Bearings 40 in the sidewalls 39 support horizontal shafts 41 and 42. A flat vane 43 is mounted to the shafts 41 within the chimneys 38. A flat vane 44 is mounted on the shafts 42 within the chimneys 38. The vanes 43 are employed to regulate the flow of gases exiting from the chimney 38 in response to thermostatic control elements (not shown) which position the shafts 41. The vanes 44 serve to regulate the flow of gas into the chimneys 38 in order to maintain a constant static pressure in the region between the vanes 44 and 43.

The regulation of the vanes 44 can be accomplished direct-1y in response to the gas pressure within the ducts 3G and 31 respectively as disclosed in the aforementioned oo-pending application S.-N. 755,048. This regulation can be accomplished by a pressure regulator device Which operates by sensing the static pressures existing between (1) a first control point located above the vane 44 and (2) a second control point located below the vane 44. Pressure taps for this regulation include a conduit 45 positioned downstream from the vane 44 and a conduit 46 positioned upstream from the vane 44. The precise control mechanism forms no part of the present invention.

According to the present invention, a permeable obstruction 47, as hereinbefore described, is positioned horizontally across the cross-section of the chimney 38 between the vanes 43 and 44.

The permeable obstruction 47 performs the same function in the mixing box chimneys 38 as hereinbefore described, i.e., a portion of the sound energy appearing as relatively low frequency sound is converted to sound energy in the form of relatively high frequency sounds. The exposed surfaces on the interior of the mixing box 32 are lined with a sound absorbing lining 48 for the hereinbefore described purpose of absorbing sound energy of the relatively high frequency sounds. The sound absorbing lining 48 may comprise any acoustical insula: tion mate-rial such as glass wool, mineral wool and the like.

In the mixing box 32 illustrated in FIGURES 3, 4 and 9, the permeable obstruction 47 performs an additional function. It will be noted by reference particularly to FIGURES 4 and 9 that the downstream pressure tap conduit 45 is positioned between the top and bottom of the permeable obstruction 47. The permeable obstruction 47 serves to minimize the turbulence of the gases in the region between the two vanes 43 and 44 whereby reliable static pressure indications are obtainable through the downstream pressure tap conduit 45. In the absence of some form of grid between the two rotatable vanes 43 and 44, the well recognized wall effects of moving gases tend to interfere with accurate pressure indication, through the downstream pressure tap conduit 45 and reduce the efficiency of static pressure control within the chimney 38. The permeable obstruction 47 may be at the level of the downstream pressure tap conduit 45 or may be positioned vertically above the conduit 45, i.e., downstream with respect thereto.

A convenient means for supporting the permeable obstruction 47 is illustrated in FIGURES 3 and 4 where a number of thin wire pins 49 extend across the chimney 38 above and below the pad of permeable obstruction 47. The bottom pins 49 are bent against the wall 39 at one end for a permanent installation. The top pins 49 may be equipped with quickly removable fastening clips 50 to permit extraction of pins 49 in the event it is desired to replace the pad of permeable obstruction 47.

A preferred material for the permeable obstructions of this invention is coarse coconut fibers coated with a film of natural rubber or synthetic rubber. Such materials, commercially available under the name- Tulatex, can be formed into batts which are effective as the permeable obstructions. A portion 51 of this material is illustrated in FIGURE 8 on an enlarged scale. This material has a plurality of interlaced coated fibers 52 which define gas passage-ways 53.

SOUND COMPARISONS Equal loudness contours have been plotted in FIG- URES 5, 6 and 7 as curves I and II. Noise intensities frequently are characterized for reference and comparison by the intensity of the sound at frequencies of 1000 cycles per second. The relative loudness of a noise is a com posite of the relative loudness of its constituentsounds. Equal loudness curves indicate the sound intensity at all frequencies required to create equivalent loudness impressions.

In general noise levels having an intensity below about 40 decibel at 1000 cycles per second are considered as quiet; noise levels having an intensity above about 60 decibel at 1000 cycles per second are considered as loud.

Curve I of FIGURES 5, 6 and 7 is an equal loudness contour having a value of 60 decibel at about 1000 cycles per second. Noise levels above curve I can be considered as loud. Curve II of FIGURES 5, 6 and 7 is an equal loudness contour having a value of 40 decibel at about 1000 cycles per second. Noise levels below curve II can be considered as quiet. Noise levels between curve I and curve 11 are in general of moderate sound intensity and will be considered as loud by some persons and considered as quiet by other persons.

To develop the comparative information for FIGURES 5, 6 and 7, air conditioning apparatus as shown in FIG- URES 3 and 4 was employed. Sound intensities were recorded at a variety of sound frequency bands under various conditions. The curve F in FIGURE 5 is a loudness contour obtained from a mechanical fan which was employed to impel conditioned air through the cool air cells 31. It will be observed that the curve F is entirely above the curve I in the low and intermediate frequency bands. Thus the noise of the fan is considered as loud.

As already discussed, fan noises are only one component of the noise level resulting at an air conditioning outlet. The actual noise level will vary according to the velocity of air flow, the quantity of air flow, and other parameters. FIGURE 6- illustrates noise levels resulting at air flows of 200 cubic feet per minute of conditioned air at 1 /2 inches (water) pressure. FIGURE 7 illustrates noise levels resulting at air flows of 300 cubic feet per minute of conditioned air at 4 inches (water) pressure.

Curve X of FIGURES 6 and 7 was obtained by using only the air discharge chimneys 38 of FIGURES 3 and 4, without the permeable obstruction 47 and without the sound-absorbing linings 48. It will be observed that the curves X represent noise levels which are characterized as loud, i.e., above the curve I at frequencies of about 1000 cycles per second.

The curves Y of FIGURES 6 and 7 represent noise levels measured by using the air discharge chimneys 38 of FIGURES 3 and 4 in conjunction with the sound-absorbing linings 48 of matted glass-fibers; no permeable obstruction 47 was included in the structure while data for the curves Y was obtained. It will be observed from the curves Y that the addition of sound absorbing surfaces in an air discharge system serves to reduce the noise level at substantially all frequencies, although the relatively high frequencies are reduced more than relatively low frequencies. This latter feature is not clearly brought out in FIGURES 6 and 7 although it will be observed that the departure between curves X and Y exhibits a tendency to increase with frequency.

The curves Z of FIGURES 6 and 7 represent the noise level recorded when the permeable obstruction 47 was placed in the air discharge chimneys 38 of FIGURES 3 and 4 in combination with sound absorbing linings 48. The specific material was a batt of Tulatex which is randomly arranged coconut fibers, individually coated with a film of synthetic rubbery material such as neoprene. The chimneys 38 were nine inches wide by three inches thick, i.e., 27 square inches in cross-section. The Tulatex batts were about %-inch thick.

It will be observed that the curves Z represent the noise level reduction achieved by the combination of the present invention.

In FIGURE 6, the curve Z is entirely beneath the equal loudness contour curve II indicating that the noise level is characterized as quiet. In FIGURE 6, the curve Z is significantly lowered to a level which is closer to curve II than to curve I. Further in FIGURE 7 the substantial reduction of intensity in the relatively low frequency sounds should be noted. The contrast in FIGURE 7 between curves Y and Z in the relatively low frequency ranges illustrates the effectiveness of the present invention in re- 6 ducing the noise level of the relatively low frequency sounds.

According to the provisions of the patent statutes, we have explained the principle, preferred construction and mode of operation of our invention and have illustrated and described what we now consider to represent its best embodiment. However, we desire to have it understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

We claim:

1. In a dual duct air conditioning system which includes separate ducts in a building for supplying separate streams of separately conditioned air to selected zones within the said building, the improvement comprising air discharge means within said zones including open ended discharge chimneys extending from each of said ducts, mixer box means surrounding said chimneys and being internally lined with sound absorbing means comprising fibrous coatings, a grid opening in said mixer box means for discharging air therefrom, first valve means and second valve means downstream therefrom in each said chimney, and a permeable obstruction within each of said chimneys between said first valve means and said second valve means, said permeable obstruction having a multiplicity of gas passageways therethrough, each of said gas passageways having a cross-section which is negligible in contrast to the cross-section of said chimney, the cumulative cross-sectional area of said gas passageways being between 50 and percent of the cross-section of said chimneys, said first valve means being adapted to regulate the flow of conditioned air through said chimney to maintain a substantially constant static pressure downstream therefrom, separate pressure sensing conduits associated with said first valve means positioned one upstream and one downstream with respect to said first valve means, said one downstream sensing conduit communicating with said chimney between said first valve means and the downstream extremity of said permeable obstruction.

2. Discharge means for releasing ventilating air from a duct into an enlarged zone including a permeable obstruction in said duct comprising a pad of fibrous material providing a multiplicity of gas passageways therethrough, said pad of fibrous material comprising coconut fibers coated with a rubbery substance, each of said gas passageways having a cross-section which is negligible in contrast to the cross-section of said duct, the cumulative cross-sectional area of said gas passageways being between about 50 and about 75 percent of the cross-section of said duct, a chamber including an inlet port and an outlet port, said inlet port being in communicating relation with the said duct downstream of said permeable obstruction, and sound absorbing means in sound absorbing relation with the gases discharged through said obstruction and said duct, said sound absorbing means being downstream from said obstruction and comprising fibrous coatings on the exposed surfaces of said chamber against which the said gases impinge.

References Cited in the file of this patent UNITED STATES PATENTS 2,001,916 Mazer May 21, 1935 2,644,389 Dauphinee July 7, 1953 2,750,865 Tutt June 19, 1956 2,896,849 Argentieri July 28, 1959 2,948,210 Conlan Aug. 9, 1960 FOREIGN PATENTS 17,441 Great Britain July 31, 1911 

